Vehicles with multiple drive wheels require a transfer case to convert a single torque source into a torque source for one or more rear differentials and one or more front differentials, each of which drives a pair of drive wheels on an axle.
In one method used in the art, a pinion differential assembly directly attached to the input shaft in the transfer case is used to divide power between the front drive shaft and the rear drive shaft. A first pinion gear on the differential is used to drive a first sprocket, which is connected to a second sprocket (and ultimately, the front drive shaft) through the high speed chain drive. A second pinion gear, located on the opposite side of the differential from the first pinion gear, is used to directly drive the rear drive shaft. The first sprocket is engaged to, or disengaged from, the first pinion gear by a first clutch mechanism. The second pinion gear is continually engaged to the rear output shaft, the disengagement of power to the rear drive wheels being accomplished by a standard transmission assembly that disengages the power to the input shaft.
In one method of "full time" four wheel drive known in the prior art, a planetary gear differential has been used to divide drive torque from the vehicle transmission between the front and rear output shafts. In such a system, an electromagnetic clutch is provided to selectively lock the planetary gear differential if a predetermined slippage is detected between the front and rear output shafts. In a normal operating condition, the electromagnetic clutch is not energized, and the sun gear and ring gear can rotate relative to each other, providing drive torque to the front and rear drive shafts. When the electromagnetic clutch is activated, the ring gear is locked relative to the sun gear sleeve, so that a direct drive condition exists between the transfer case input shaft and each of the front and rear output shafts, since the planet gears are unable to rotate about their associated shafts, thereby preventing rotation of the planet carrier, which is locked to the transfer case input shaft.